Device and method for preheating combustibles in combined gas and steam turbine installations

ABSTRACT

The invention relates to a device for preheating combustibles in combined gas and steam turbine installations. Said device comprises at least one steam generator ( 5, 6, 7 ) for heating, evaporating and superheating feed water, which has a feed water preheater ( 15, 16, 43, 51 ), an evaporator ( 8, 9, 10 ) and a superheater ( 11, 12, 13 ), at least part of the heated feed water being used for preheating combustibles in a combustible preheater, which can be supplied with feed water of a predetermined temperature by a feed water supply line ( 35, 36, 37, 56, 58, 60, 73 ) Once the combustible has been heated, the cooled feed water can be drained using a feed water drain line ( 57, 59, 61, 74 ). The aim of the invention is to configure a device of this type so that it can be used in different gas turbine installations and its heating capacity can be rapidly and cost-effectively adapted to predetermined combustible preheating requirements. To achieve this, the combustible is preheated in stages ( 48, 49, 50 ), which can be activated individually as required and through which the combustible can successively pass.

[0001] The invention relates to a device for fuel preheating in combinedgas and steam turbine plants, with at least one steam generator for theheating, evaporation and superheating of feedwater, said steam generatorhaving a feedwater preheater or condensate preheater, an evaporator anda superheater, at least part of the heated feedwater being capable ofbeing used for the preheating of fuel, and the fuel preheater beingcapable of being supplied with feedwater having a predeterminedtemperature by means of a feedwater delivery line, and, after theheating of the fuel, the cooled feedwater being capable of beingdischarged by means of a feedwater discharge line.

[0002] For the provision of steam for driving a steam turbine, feedwateris heated, evaporated and superheated under high pressure. This takesplace, as a rule, in a plurality of steam generator stages, as a rule alow-pressure steam generator stage, a medium-pressure steam generatorstage and a high-pressure steam generator stage. The steam generatorstages consist of a feedwater preheater or condensate preheater, inwhich the feedwater, which is supplied by a feedwater pump, is heated toboiling temperature before entry into a drum assigned to the respectivepressure stage and is partially also evaporated, of an evaporator whichis connected to the respective drum and in which saturated steam isgenerated, and of a superheater, connected to the steam region of thedrum, for supplying heat to the saturated steam, without an increase inthe pressure of the saturated steam, which is then subsequentlyconducted to a part of the steam turbine which corresponds to therespective steam pressure (low-pressure, medium-pressure andhigh-pressure turbine part). In a combined gas/steam turbine plant, thesupply of heat to the steam generator stages takes place by means of thehot exhaust gases from a gas turbine. In addition, in themedium-pressure steam generator stage, the exhaust steam obtained from ahigh-pressure turbine part is admixed, in turn, to the superheatedmedium-pressure steam and is heated once more in an intermediatesuperheater, in order thereafter to be supplied to the medium-pressureturbine part. Alternatively to delivery to the corresponding steamturbine part, the steam may also be conducted into a steam turbinecondenser, from where the water/steam circuit is closed by a condensatepump pumping to the condensate preheater again the condensate which isformed in the condenser. For the provision of energy for generating gasfor the gas turbine or additional heat for the steam generators of thesteam turbine in a combined gas/steam turbine plant, fuel, for examplenatural gas, is burnt. In order to make this combustion effective, thefuel is previously heated. The preheating of the fuel takes place bymeans of a fuel preheater. For this purpose, for example, part of thealready heated medium-pressure feedwater is branched off downstream ofthe medium-pressure feedwater preheater, before entry into themedium-pressure drum, is led past the fuel by the heat exchanger methodand is delivered to the water/steam circuit again between the condensatepump and the condensate preheater. In this case, the fuel reachestemperatures of up to 200° C. Alternatively, medium-pressure feedwateris branched off even before entry into the medium-pressure feedwaterpreheater, fuel temperatures of about 140-160° C. being reached.

[0003] One disadvantage of this is that the fuel preheater must have ahigh pressure rating, with the result that it is sensitive towardtemperature changes and is costly to produce and maintain. Moreover,even in modern gas turbine plants, the possible preheating of the fuelup to 200° C. can scarcely be utilized, since, in the case of higherpreheating, there is, for example, the problem of what may be referredto as “combustion chamber humming”, so that it has hardly been possiblehitherto to use the installed costly preheating systems.

[0004] The object of the present invention is, therefore, to deliver adevice for fuel preheating of the type described above, which can beused in different gas turbine plants, and which can be adapted quicklyand cost-effectively in terms of its heating capacity to a predeterminedfuel preheating requirement.

[0005] The object is achieved in that fuel preheating takes place infuel preheating stages which can be activated individually, as required,and through which the fuel can be conducted in succession.

[0006] By means of a device of this type, a high degree ofstandardization, along with a repetition effect, can be achieved inplanning and implementation. Owing to the buildup of individual fuelpreheating stages capable of being used, as required, varioustemperature ranges can be set in stages, depending on the arrangementand number of fuel preheating stages and their connection to the steamgenerator or steam generators. A modular construction is thus possible,so that the device can be used for various gas/steam turbine plants orcan be adapted by means of simple modifications (modular effect).

[0007] If fuel preheating takes place in a first stage (for example withcondensate recirculation), then complete utilization of the feedwaterquantity for fuel preheating is achieved when the feedwater preheater isconstructed from two parts which are connected in parallel to theexhaust gas and the fuel preheating stage is connected between theparts, the feedwater delivery line leading from an outlet of the firstpart of the feedwater preheater to the fuel preheating stage, and thefeedwater discharge line leading from the fuel preheating stage to aninlet of the second part of the feedwater preheater. In this device, theentire feedwater quantity first runs through part of the feedwaterpreheater, is heated and then flows through the fuel preheating stage,in which it gives off the previously absorbed heat completely orpartially. Subsequently, the cooled feedwater runs through the secondpart of the feedwater preheater, in which it is heated again. There aretherefore no losses due to throttlings which would take place iffeedwater were branched off. Only relatively low pumping capacities arenecessary. Advantageously, there is no need for any additionalcirculation of feedwater. The efficiency of the fuel preheating stage isthus correspondingly high.

[0008] As-required adaptation to the desired temperature of the fuelduring fuel preheating is possible when a fuel preheating stage isassigned to a condensate preheater or to a medium-pressure feedwaterpreheater or to a high-pressure feedwater preheater. Thus, the differentpressure and temperature ranges of the feedwater preheating stages canat the same time be utilized, without additional measures, for fuelpreheating. In particular, the various stages may be combined with oneanother according to the heating requirement.

[0009] A particularly advantageous utilization of the feedwater energyin the steam generator is afforded when three fuel preheating stages areprovided, a first fuel preheating stage being assigned to a condensatepreheater, a second fuel preheating stage being assigned to amedium-pressure feedwater preheater and a third fuel preheating stagebeing assigned to a high-pressure feedwater preheater. The fuel canthereby be heated carefully, in stages, to a very high temperature. Atthe same time, it is necessary for the fuel preheating stage to haveonly one design which is adapted to the respectively prevailing pressureand temperature. Thus, in the case of the lower pressure stages, a lesscomplicated design is possible with the result that safety is notrestricted and, moreover, costs can be saved.

[0010] When the feedwater delivery line of a fuel preheating stage isattached to the feedwater pump downstream of the condensate preheaterand the feedwater discharge line of the fuel preheating stage leadsupstream of the condensate preheater, an additional condensaterecirculation pump is not necessary on account of the pressure which isexerted by the feedwater pump.

[0011] There is no need for throttling to a low pressure level, forexample about 4-5 bar, when the feedwater delivery line of a fuelpreheating stage is attached downstream of a feedwater preheater and thefeedwater discharge line of the fuel preheating stage leads upstream ofthe feedwater pump.

[0012] Flashings are insignificant when the feedwater delivery line of afuel preheating stage is attached between the medium-pressure drum andthe medium-pressure feedwater preheater and the feedwater discharge lineof the fuel preheating stage leads to the low-pressure drum.

[0013] When the first and/or the second and/or the third fuel preheatingstage are connected in each case between the two parts of the condensatepreheater and/or the two parts of the medium-pressure feedwaterpreheater and/or the two parts of the high-pressure feedwater preheater,then, advantageously, no additional feedwater has to be circulated.

[0014] A slower startup with a lower temperature gradient is ensuredwhen an admixing line, starting between the condensate pump and ashutoff valve, leads to the inlet of the feedwater into the fuelpreheating stage. The quantity of admixed cooler feedwater can be setcontinuously, with the result that the temperatures are regulated.

[0015] A regulated startup in the medium-pressure fuel preheater isafforded when an admixing line is attached to an inlet of the fuelpreheating stage by means of a medium-pressure feedwater delivery line,the admixing line starting between the feed pump and the medium-pressurefeedwater preheater.

[0016] The aim of the invention is, furthermore, to improve a method forfuel preheating in combined gas and steam turbine plants, feedwater inat least one steam generator being heated in a feedwater preheater orcondensate preheater, being evaporated in an evaporator and beingsuperheated in a superheater, at least part of the heated feedwaterbeing used for the preheating of fuel in a fuel preheater which issupplied with feedwater having a predetermined temperature by means of afeedwater delivery line, and, after the heating of the fuel, the cooledfeedwater being discharged in each case by means of a feedwaterdischarge line.

[0017] The object, therefore, is to provide a method for fuel preheatingin combined gas and steam turbine plants, which can be used in differentgas turbine plants and in which the heating capacity of the fuelpreheater can be adapted quickly and cost-effectively to a predeterminedfuel preheating requirement.

[0018] The object directed at a method is achieved in that fuelpreheating takes place in fuel preheating stages which can be activatedindividually, as required, and through which the fuel is conducted insuccession. By the fuel being heated in stages, a flexible and virtuallyoptimum setting of the fuel preheating temperatures to the requirementsof the plants is possible and at the same time is cost-effective becausenot all the components used have to satisfy all the pressure andtemperature requirements at the same time, but only those requirementsactually prevailing locally in each case.

[0019] When the feedwater is conducted first through one part of thefeedwater preheater, subsequently to the fuel preheating stage andthrough a second part of the feedwater preheater, the two parts beingconnected in parallel to the exhaust gas, the feedwater is first heateddirectly to the desired temperature and, subsequently, the entire heatedfeedwater quantity can be utilized for heating the fuel. Thereafter, thecooled feedwater is heated again in the second part of the feedwaterpreheater. There are therefore no pressure losses on account ofthrottlings or possible heat losses due to prolonged diversion.

[0020] When feedwater from a condensate preheater is conducted through afirst fuel preheating stage, feedwater from a medium-pressure feedwaterpreheater is conducted through a second fuel preheating stage andfeedwater from a high-pressure feedwater preheater is conducted througha third fuel preheating stage, the individual temperature and pressurestates of the feedwater are utilized particularly advantageously and theindividual fuel preheating stages have to be geared in each case only tothe requirements of one stage, with the result that costs can be saved(modular construction).

[0021] Exemplary embodiments of the invention are given in the figuresof which:

[0022]FIG. 1: shows a diagrammatic illustration of a gas and steamturbine plant,

[0023]FIG. 2: shows a fuel preheating stage,

[0024]FIG. 3a: shows two fuel preheating stages,

[0025]FIG. 3b: shows a detail of the water/steam separation device,

[0026]FIG. 3c: shows two fuel preheating stages with alternative linerouting,

[0027]FIG. 4: shows two fuel preheating stages, and

[0028]FIG. 5: shows three fuel preheating stages.

[0029]FIG. 1 shows a diagrammatic illustration of a gas and steamturbine plant. The steam turbine 1 is constructed from three turbinestages 2, 3, 4, to be precise a low-pressure steam stage 2, amedium-pressure steam stage 3 and a high-pressure steam stage 4. Theturbine stages 2, 3, 4 are supplied with steam by means of three steamgenerators 5, 6, 7, to be precise a low-pressure steam generator 5, amedium-pressure steam generator 6 and a high-pressure steam generator 7.

[0030] The steam generators 5, 6, 7 are basically of similarconstruction. The steam generators 5, 6, 7 consist in each case of anevaporator 8, 9, 10, of a superheater 11, 12, 13, of a condensatepreheater 42 or a feedwater preheater 15, 16 and of a drum 17, 18, 19 inwhich feedwater condensate and feedwater steam collects, and also of aheat source 20 which, in the present case, is fed by exhaust gases froma gas turbine 21.

[0031] The feedwater is first preheated in a condensate preheater 42.Thereafter, part of the feedwater is branched off and is led through alow-pressure feedwater delivery line 35 to a low-pressure drum 17.Another part of the feedwater is led from a feedwater pump 24, via amedium-pressure feedwater delivery line 36, to a medium-pressure drum 18and, via a high-pressure feedwater delivery line 37, to a high-pressuredrum 19. From the drums 17, 18, 19, saturated steam is first generatedfrom the feedwater in the low-pressure evaporator 8, in themedium-pressure evaporator 9 or in the high-pressure evaporator 10. Thisis then followed, in the low-pressure steam generator 5, by thelow-pressure superheater 11, in which the saturated steam is heated tothe desired superheating temperature. From the low-pressure superheater11, a low-pressure turbine delivery line 25 leads the steam to thelow-pressure turbine stage 2 or, in a bypass mode, through alow-pressure steam bypass line 40 into a turbine condenser 23, in whichthe steam is condensed again and is conducted by means of a condensatepump 41 to the condensate preheater 42. In the medium-pressure steamgenerator 6, the feedwater is first heated in the medium-pressurefeedwater preheater 15, and, subsequently, saturated steam is generatedin the medium-pressure evaporator 9, which is then conducted to themedium-pressure superheater 12. The superheated steam is subsequentlyfed into a high-pressure turbine discharge line 28 which carries steamwhich has emerged from the high-pressure turbine part 4 and which then,together with the medium-pressure steam, is further superheated in amedium-pressure intermediate superheater 22 and is subsequentlyconducted, via a medium-pressure turbine delivery line 26, to themedium-pressure steam stage 3 of the steam turbine 1. Alternatively, themedium-pressure steam may be supplied to the turbine condenser 23 via amedium-pressure steam bypass 39. The feedwater supplied to thehigh-pressure steam generator 7 is heated first in a high-pressurefeedwater prehater 16 and thereafter in a further high-pressurefeedwater preheater 43. The saturated steam generated in thehigh-pressure evaporator 10 is superheated in the two stages of thehigh-pressure superheater 13 and is supplied to the high-pressure steamstage 4 of the steam turbine 1 via a high-pressure turbine delivery line27. Alternative steam conduction may take place via the high-pressuresteam bypass 29 which connects the high-pressure turbine delivery line29 to the high-pressure turbine discharge line 28. Drum water can bedischarged from all the drums 17, 18, 19 into a common tank 38 vialow-pressure drum discharge lines 32, medium-pressure drum dischargelines 33 and high-pressure drum discharge lines 34. Moreover, thesuperheated medium-pressure steam and the superheated high-pressuresteam can be cooled by feedwater extracted downstream of the feedwaterpump being supplied in each case into a medium-pressure steam deheater30 or high-pressure steam deheater 31 (injection cooler) builtrespectively into the medium-pressure turbine delivery line 26 or thehigh-pressure turbine delivery line 27.

[0032] Moreover, electrical valves or flaps are installed in most lines,so that the steam and feedwater flows can be controlled.

[0033] For generating the gases for operating the gas turbine 21, fuel,for example natural gas or fuel oil, is required. For fuel preheating,heated feedwater from the medium-pressure steam generator 6 is used inthe prior art, as illustrated in FIG. 1. After preheating in themedium-pressure feedwater preheater 15, the feedwater is conducted to afuel preheater 44. In the fuel preheater 44, the heat energy from thefeedwater is transmitted to the fuel in countercurrent on the heatexchanger principle. Subsequently, the cooled feedwater is fed into thefeedwater circuit again upstream of the condensate preheater 42. Duringfuel heating, the feedwater is cooled to 60-80° C., while the fuel isheated up to 200° C. This high preheating cannot always be utilized,since, in the case of higher preheating of the fuel, problems, such as,for example, what may be referred to as “combustion chamber humming”,may arise during combustion in the combustion chamber of the gas turbine21.

[0034]FIG. 2 shows a fuel preheating stage 48 according to theinvention. By means of a low-pressure feedwater delivery line 58 whichextracts low-pressure feedwater from a tap of the high-pressure feedpump, the feedwater heated by the condensate preheater 42 is conductedto the fuel preheating stage 48. After the heating of the fuel in theheat exchanger in the fuel preheating stage 48, the feedwater isintroduced upstream of the condensate preheater 42 through alow-pressure feedwater discharge line 59. Valves 63 with an electronicactuating drive are provided in the low-pressure feedwater delivery line58, so that the run of the feedwater can be shut off or set. Tosafeguard against destruction due to excess pressure, a pressure reliefvalve 45 is provided. For careful startup with a lower temperaturegradient, an admixing line 46 is provided. The admixing line 46 startsupstream of the condensate preheater 42, even upstream of theintroduction point 65 of the feedwater cooled for fuel preheating. Saidadmixing line ends upstream of the fuel preheating stage 48 and has aregulatable valve 64. Moreover, the entire fuel preheating stage 48,together with the low-pressure feedwater delivery line 58, withlow-pressure feedwater discharge line 59 and with the admixing line 46,is provided as a retrofit system, so that it can be used, as required.Parallel to the low-pressure feedwater delivery line 58 and thelow-pressure feedwater discharge line 59 and to the fuel preheatingstage 48, a parallel feedwater line 66 runs from the feedwater pump 24to upstream of the condensate preheater 42. Set up in this parallelfeedwater line 66, in addition to a further valve 68, is a throttlediaphragm 47, with the aid of which the higher feedwater pressure (inthe case of a bypassed fuel preheater) is reduced to the level of thecondensate system.

[0035]FIG. 3a shows an extract of a circuit diagram of a gas and steamturbine plant with two fuel preheating stages 48, 49. The low-pressurefuel preheating stage 48 is attached, as described in FIG. 2. The fuelheated in the fuel preheating stage 49 is subsequently fed to a secondfuel preheating stage, the medium-pressure preheating stage 49. This issupplied by means of a medium-pressure feedwater delivery line 56 withfeedwater which is extracted downstream of the medium-pressure feedwaterpreheater 15, 51. The medium-pressure feedwater preheater 15, 51 is inthis case constructed from two parts 15 and 51 connected in series. Theintroduction of cooler feedwater, which is extracted usptream of thefirst medium-pressure feedwater preheater 15, upstream of the secondmedium-pressure feedwater preheater 51 prevents flashing in the secondmedium-pressure feedwater preheater 51. This may be relevant, inparticular, in the case of a relatively low throughflow, for examplewhen the medium-pressure fuel preheating stage 49 is not in operation.

[0036] A throttle diaphragm 54 is provided between the medium-pressuredrum 18 and the start of the medium-pressure feedwater delivery line 56.The aim of this device is to avoid flashings in the medium-pressurepreheater 51 due to pressure buildup during steam formation.Furthermore, there is a steam-bubble separator 71, the functioning ofwhich is explained in more detail in the illustration of a detail inFIG. 3b. Upstream of the fuel preheating stage 49, via a three-way valve68, the delivery of colder feedwater via a delivery line 69 issuingupstream of the medium-pressure feedwater preheater 15 is possible forthe purpose of a careful startup. The discharge of the cooled feedwaterdownstream of the fuel preheating stage 49 takes place via amedium-pressure feedwater discharge line 57 which issues upstream of thefeedwater pump 24. This medium-pressure feedwater discharge line 57 hasa pressure relief valve 53 for safeguarding against pressure excess.Pressure rating has to take place merely in terms of condensatepreheating pressure, for example 25-30 bar. If no fuel preheating isdesired, the feedwater delivery lines to the fuel preheating stages 48,49 may be shut off, as required, by means of plurality of valves 70.

[0037]FIG. 3b shows an illustration of the steam bubble separator 71 inthe form of a detail. Flashings may occur, for example, due to the factthat feedwater is excessively supersaturated with steam, for examplewhen the feedwater temperature or the pressure falls. By means of thesteam-bubble separator 71, the flow of the feedwater flowing in from themedium-pressure feedwater preheater 51 is slowed, and steam bubblespossibly present rise vertically upward, from where they are thenentrained to the medium-pressure drum 18. The feedwater without bubblescollects in the lower steam-bubble separator part 72 and is drawn offhere, via the medium-pressure feedwater delivery line 56, to the fuelpreheating stage 49.

[0038]FIG. 3c shows an extract from a circuit diagram of a gas and steamturbine plant with two fuel preheating stages 48, 49. The first fuelpreheating stage 48 corresponds to that in FIG. 3a. The medium-pressurefeedwater discharge line 61/62 of the second fuel preheating stage 49runs to the low-pressure drum 17. This results in less significantflashing problems, because the feedwater is still very hot.

[0039]FIG. 4 shows an extract from a circuit diagram of a gas and steamturbine plant with two fuel preheating stages 48, 49. Themedium-pressure feedwater preheater is constructed from two parts 15, 51which are connected in parallel to the exhaust gas. The fuel preheatingstage 49 is connected between the parts 15, 51 of the medium-pressurefeedwater preheater, the feedwater delivery line 73 leading from anoutlet of the first part 15 of the feedwater preheater to the fuelpreheating stage 49, and the feedwater discharge line 74 leading fromthe fuel preheating stage 49 to an inlet of the second part 51 of thefeedwater preheater. Provided parallel to the fuel preheating stage 49is a bypass line 75, through which the feedwater can be conducted, in sofar as it is not to flow through the fuel preheating stage 49.

[0040]FIG. 5 shows an extract from a circuit diagram of a gas and steamturbine plant with three fuel preheating stages 48, 49, 50. The firstfuel preheating stage 48 is fed by feedwater from the region upstream ofthe medium-pressure feedwater preheater 15. The second fuel preheatingstage 49 is connected between the two parts 15 and 51 of themedium-pressure feedwater preheater, and the third fuel preheating stage50 is connected between the two parts 16 and 43 of the high-pressurefeedwater preheater. The fuel is in this case heated in the first stage48 to approximately 130-150° C. It is heated in the second stage 49 to200-220° C. and in the third stage 50 to approximately 300° C. The fuelpreheaters are in each case capable of being bridged by feedwater lines80, 81, 82 and can thus be bypassed, if this is necessary.

1. A device for fuel preheating in combined gas and steam turbineplants, with at least one steam generator (5, 6, 7,) which has afeedwater preheater (15, 16, 43, 51), an evaporator (8, 9, 10) and asuperheater (11, 12, 13), and with a number of fuel preheating stages(48, 49, 50) which can be activated individually, as required, and whichare arranged in series, at least part of heated feedwater being capableof being supplied to the fuel preheating stages (48, 49, 50) by means ofa feedwater delivery line (35, 36, 37, 56, 58, 60, 73) and being capableof being discharged from the fuel preheating stages (48, 49, 50) bymeans of a feedwater discharge line (57, 59, 61, 74), characterized inthat the device comprises three fuel preheating stages (48, 49, 50), bymeans of the feedwater delivery line (35, 36, 37, 56, 58, 60, 73) afirst fuel preheating stage (48) being connected to a condensatepreheater (42), a second fuel preheating stage (49) to a medium-pressurefeedwater preheater (15, 51) and a third fuel preheating stage (50) to ahigh-pressure feedwater preheater (16, 43).
 2. The device as claimed inclaim 1, characterized in that the medium-pressure feedwater preheateris constructed from two parts (15, 51) which are connected in parallelto the exhaust gas and the second fuel preheating stage (49) isconnected between the parts (15, 51), the feedwater delivery line (73)being connected to an outlet of the first part (15) of the feedwaterpreheater and to the second fuel preheating stage (49), and thefeedwater discharge line (74) being connected to the second fuelpreheating stage (49) and to an inlet of the second part (51) of thefeedwater preheater.
 3. The device as claimed in claim 1 or 2,characterized in that the feedwater delivery line (58) of the first fuelpreheating stage (48) is attached to a feedwater pump (24) downstream ofthe condensate preheater (42), and the feedwater discharge line (59) ofthis first fuel preheating stage (48) leads upstream of the condensatepreheater (42).
 4. The device as claimed in claim 3, characterized inthat the feedwater delivery line (56) at the second fuel preheatingstage (49) is attached downstream of the medium-pressure feedwaterpreheater (15, 51), and the feedwater discharge line (57) of this secondfuel preheating stage (49) leads upstream of the feedwater pump (24). 5.The device as claimed in one of claims 1 to 3, characterized in that thefeedwater delivery line (56) of the second fuel preheating stage (49) isattached between a medium-pressure drum (18) and the medium-pressurefeedwater preheater (15, 51), and the feedwater discharge line (60) ofthis second fuel preheating stage (49) is connected to a low-pressuredrum (17).
 6. The device as claimed in one of claims 1 to 3,characterized in that the feedwater delivery line (56) of the secondfuel preheating stage (49) is attached between a medium-pressure drum(18) and the medium-pressure feedwater preheater (15, 51), and thefeedwater discharge line (60) of this second fuel preheating stage (49)is attached between the condensate preheater (42) and a low-pressuredrum (17).
 7. The device as claimed in one of claims 1 to 6,characterized in that the condensate water preheater (42) and/or themedium-pressure feedwater preheater (15, 51) and/or the high-pressurefeedwater preheater (16, 43) are designed in two parts, and the firstfuel preheating stage (48) is connected between the two parts of thecondensate water preheater (42) and/or the second fuel preheating stage(49) is connected between the two parts of the medium-pressure feedwaterpreheater (15, 51) and/or the third fuel preheating stage (50) isconnected between the two parts (16, 43) of the high-pressure feedwaterpreheater (16, 43).
 8. The device as claimed in one of claims 1 to 7,characterized in that an admixing line (46) is attached between acondensate pump (41) and a shutoff valve (76) and leads to an inlet (77)of the feedwater into the first fuel preheating stage (48).
 9. Thedevice as claimed in one of claims 3 to 8, characterized in that afurther admixing line (78) is attached to a further inlet (79) of thesecond fuel preheating stage (49) by means of a medium-pressurefeedwater delivery line (56), the further admixing line (78) startingbetween the feed pump (24) and the medium-pressure feedwater preheater(15).
 10. A method for fuel preheating in combined gas and steam turbineplants, in particular with the aid of a device as claimed in one ofclaims 1 to 9, feedwater in at least one steam generator (5, 6, 7) beingheated in a feedwater preheater (15, 16, 43, 51) or condensate preheater(42), being evaporated in an evaporator (8, 9, 10) and being superheatedin a superheater (11, 12, 13), at least part of the heated feedwaterbeing used for the preheating of fuel in a fuel preheater which issupplied with feedwater of a predetermined temperature by means of afeedwater delivery line (35, 36, 37, 56, 58, 60, 73), and, after theheating of the fuel, the cooled feedwater being discharged in each caseby means of a feedwater discharge line (57, 59, 61, 74), characterizedin that fuel preheating takes place in fuel preheating stages (48, 49,50) which can be activated, as required, the fuel being conducted insuccession through the fuel preheating stages, and in that feedwaterfrom a condensate preheater (42) is conducted through a first fuelpreheating stage (48), feedwater from a medium-pressure feedwaterpreheater (15, 51) is conducted through a second fuel preheating stage(49) and feedwater from a high-pressure feedwater preheater (16, 43) isconducted through a third fuel preheating stage (50).
 11. The method asclaimed in claim 10, characterized in that the feedwater is conductedfirst through a first part (15) of the medium-pressure feedwaterpreheater, subsequently through a feedwater delivery line (73) to thesecond fuel preheating stage (49) and thereafter through a feedwaterdischarge line (74) to a second part (51) of the medium-pressurefeedwater preheater, the two parts (15, 51) being connected in parallelto the exhaust gas.